This invention relates to a system for identifying documents and, more particularly, to a system for validating paper currency.
Apparatus for determining the validity of proffered paper currency, either as part of a stand-alone paper currency changer or as past of a vending machine, are well known in the art. Currency validators of this sort are shown in Collins U.S. Pat. No. 4,588,292 and British Patent Specification No. 2,088,051. Typically, such validators move the article of currency past a magnetic or optical sensor to obtain a set of successive sample values indicating the magnetic permeability or optical reflectance of the article of currency along a strip portion. The sample values are compared with a set of reference values indicating the same property of an actual or hypothetical genuine article of currency along that strip, and an indication of the genuineness of the proffered currency is generated as a function of the result of the comparison. In multiple-denomination versions of such validators, the sample values are compared with a plurality of sets of reference values, one set for each denomination, to assign a denomination on the basis of the best match.
Currency validators of the type described above operate on the assumption that the pattern being matched is a fixed distance from the leading edge of the bill, so that the detector signal can be sampled at the proper time on the basis of a leading-edge signal. In practice, the timing of such leading edges is difficult to establish accurately. Even if the timing of such leading edges could be established accurately, the printing process itself introduces variations in the displacement between the leading edge of the sheet and the printing area. Moreover, through wear, a bill can change in length, so that distances between portions of a sample bill along the scan path do not equal the distance between corresponding portions of a reference bill.
Similar types of errors may arise when the sample and reference values are analog signals representing a continuously variable quantity such as reflectance. For various reasons such as aging of the document or degradation of the reflectance sensor, the average of the sample values may vary considerably from the average of the stored reference values. In such an instance, even if the variations in the sample values track those of the reference values, the differences in average value may lead to false rejections of genuine documents. Alternatively, such amplitude offsets between the sample values and the corresponding reference values may result in false acceptances if the sensitivity of the system is lowered to reduce the rate of false rejections.
Because of these variations and sensor deficiencies, prior-art bill validators, when adjusted to reject spurious bills, also tend to reject a large proportion of genuine bills proffered.